1. Field of the Invention
The present invention relates to a system for moving object detection in a moving picture, or more in particular to a system capable of detecting a moving object in a moving picture quickly and accurately from the coded moving picture information.
2. Description of the Related Art
Conventionally-proposed moving object detection systems include a method in which a background picture is stored in advance and the difference is taken between the background picture and an object picture thereby to detect that the object picture is a moving picture, and a method in which the direction and magnitude of the movement of each pixel between frames are calculated so that the portion for which the motion is calculated is detected as a motion area. In each of these methods, an appropriate algorithm is applied directly to the moving picture data to determine a moving object area.
In another conventional system for moving object detection, the moving picture data is not directly used for the moving object detection process, but a moving object is detected directly from the compressed moving picture information. In this method, the compressed moving picture data are not decoded for completely reproducing the moving picture, but only the information required for the detection of the moving object area is extracted from the compressed moving picture information, and the detection process is implemented on the basis of the extracted information. Conventionally, Mabuchi, Inagaki, et al. have proposed "Extraction of Dynamic Area Using MPEG Data", Institutes of Electronics, Information and Communication Engineers of Japan, TECHNICAL REPORT OF IEICE (IE-96-25).
According to this method, each block in a picture is checked as to whether it has the motion vector information used for the motion compensation. In the presence of a motion vector in a block, it is judged that a moving object exists in the particular block. This motion vector detection is accomplished by block.
In the former one of the two methods mentioned above, all the moving picture data are processed for moving object detection. Specifically, when detecting a moving object from the moving picture data stored in compressed data form, the picture is restored by decoding and then the moving object is detected. The problem, therefore, is a long processing time and a large amount of processing required for moving object detection. Also, in the case where a motion picture is compressed while a moving object is detected, both a equipment for moving object detection and a equipment for compression are required, resulting in a system very bulky as a whole.
In the latter method, on the other hand, a simple moving picture produced by computer graphics is used for simulation in the paper, and therefore the background pixels are uniformly painted. Thus, there is no possibility of a motion vector being generated in the background portion at the time of coding, with the result that a moving object can be easily and accurately detected.
In an actual motion picture of real world, however, various external factors including air fluctuations and meteorological conditions such as wind substantially eliminate a completely stationary state in which all things are under constant change. This change of state is picked up as a change at the time of compression coding, and is added to a particular block as motion vector information for motion compensation. Also, as for a block having substantially uniform pixel data arranged therein, the motion vector often changes at random in both angle and size in the process of compression coding. In such a case, even if the particular portion is included in the background, the block is erroneously detected as a moving object portion. In other words, in a picture of the actual world, a moving object cannot be detected accurately simply according to the presence or absence of a motion vector.
Further, the coded data of a moving picture by simulation uses only the forward prediction for motion compensation of each block. The coding system, however, also involves both a backward or a bidirectional prediction. No detection method for such a case is not described at all.
Also, there are some blocks which are coded with the information in the frame, i.e. intra-coded. They usually have no motion vector. In such a case, the problem is that a moving object cannot be detected for the particular block.